Variable Voltage / Variable Freq. Soft Starter 1

[duki]

Does anyone know of any manufacturer of a variable voltage/ variable frequency soft starter in the medium voltage range, 5.5, 6.6kV? Are there more advantages in using a 50% rated VFD for soft starting duties than using a real soft starter that varies the voltage and limits the current?

 

[Marke]

Hello duki

A soft starter controlls voltage and does not control frequency. A VFD controls both voltage and frequency.

There are a number of manufacturers of MV soft starters, Motortronics, Benshaw, AuCom, Solcon and others. There are also a number of manufacturers of MV VFDs, ABB, Toshiba Fuji, Rubicon and others.

One advantage of the soft starter, is price, another is eficiency, another is load torque capabilities.
Generally, if you do not need to vary the speed in operation, i.e. fixed speed operation, then look at a soft starter. If you gain an advantage from varying the speed, then look at a VFD. There are times when the motor start characteristics are ery bad, and the supply is very weak, when there is an advantage in using a VFD to start the motor. This is because the motor never operates in high slip with a VFD and so the start currents are very low compared to a reduced voltage starter.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[CJCPE]

I doubt that there is such a thing as “a 50% rated VFD for soft starting duties.” It may be possible to construct such a thing, but it doesn’t seem likely that it would offer any advantages. It would need to be bypassed after performing whatever function it might be able to perform.

A VFD can be used for starting a very large inertia requiring a longer ramp-up time than would be acceptable using other methods. An application requiring frequent starting and stopping might be another good VFD application.

 

[DickDV]

It is conceivable that a 50% rated VFD could be used just for starting purposes with little or no load on the motor other than that needed for acceleration. Such a VFD would have to be bypassed for full-speed full-load operation.

At these voltages, I'm not sure there would be any significant difference in VFD costing between the 50% load and 100% load units. The bypass would, of course, have considerable cost, as well.

About the best you can acheive with a reduced voltage starter is to reduce the inrush currents to one-half the across-the-line value. That would leave consider inrush current and the reduced voltage softstarter would have to be sized to handle this inrush current. On the other hand, an inverter could start the same load with no inrush at all, in most cases.

So, if you are soft-starting simply to reduce shock levels and mechanical stresses in the load, that can be acheived most economically with a reduced voltage starter. If you are trying to start with minimum inrush, then the VFD will give the best result.

 

[jraef]

Variable Voltage - Variable Frequency soft start? Wouldn't that actually be a description for a VFD? The main conceptual difference between a soft starter and a VFD is that a soft starter is fixed frequency, a VFD is variable. Everything else is just the details on how to get there.

I'll confirm the issue of a "50% rated VFD" being an unlikely savings. The biggest cost on MV VFDs in in making the initial leap to MV. After that, current ratings make little difference in the overall price of the VFD part. Most of the VFD power systems are really sized in big "chunks" such as 200A, 400A, 600A etc. The difference for instance in a 200A vs a 400A is usually only a few thousand dollars (<5%). The associated switchgear would be more expensive, but you would have no savings there because at some point you still need to run the motor through it, so the switchgear is the same, or in fact more because of the transfer components. The only time it makes some sense is when you have multiple motors and you want to sequentially start them with the VFD, then switch them over to ATL (DOL). Still, in that case it makes better economic sense to use a fully rated VFD and run the last motor in the sequence directly from it.

Why don't you post more details as to what problem(s) you are trying to overcome and maybe we can offer better suggestions?



JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Will a VFD synchronize with the line, or must we add a synchronism check relay to verify phase relationships before closing a bypass breaker on a VFD? Can we close a bypass breaker on a VFD or must it be an open transition?
respectfully

 

[jraef]

There are VFD suppliers who have synchronous transfer systems, but it isn't something to be taken lightly. The consequences of doing it wrong are disasterous to the drive. Robicon has been the powerhouse in that realm for years and I have seen their system work so I would trust them to know what they are doing. Most people opt for an open transition and suffer or risk the torque spike that can occur. ABB was one of the first to offer a canned "macro" for doing this with their ACS drives when they first came out, but they were always doing it open-transition on VT pumps so it wasn't a big problem. The Robicons I witnessed were on 5kV 4000HP crude oil pumps at a pipeline pump station and they were a thing of beauty. Were it not for the sound of the vacuum contactors clunking you would have no idea the transfer took place.

I've heard of people overspeeding the motor with the drive just prior to an open-transition in order to have the motor speed at reclosing be as close as possible to rated slip speed, but that's not 1st hand knowledge for me.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376